Process and apparatus for filling and orienting dry, hard ferromagnetic powders into molds



May 23, 1961 E. SCHWABE 2,984,866

PROCESS AND APPARATUS FOR FILLING AND ORIENTING DRY, HARD FERROMAGNETIC POWDERS INTO MOLDS Filed June 4, 1959 United States Patent PROCESS AND APPARATUS FOR FILLING AND ORIENTING DRY, HARD FERROMAGN'ETIC POWDERS INTO MOIJDS Eberhard Schwabe, New Brunswick, N.I., assignor to Steatite Research Corporation, Keasbey, N .J., a corporation of Delaware Filed June 4, 1959, Ser. No. 818,165

3 Claims. (Cl. 18-5) This invention relates to a method and apparatus for dry feeding powdered, hard magnetic ferrites to molds and for molding the same.

Among the objects of the invention is to provide a feeding and molding process for dry powdered hard ferromagnetic powders to form the same to shaped bodies which produces products which are at least equivalent to the products formed by wet molding processes.

It is well-known that the quality of barium ferrite or similar hard ferrite magnets is improved when the powdered ferrites are pressed to shape in a magnetic field. The magnetic field aligns the single-domain particles, thereby producing a magnetic anisotropy in the pressed pieces which remains after the sintering process. The magnetic anisotropy greatly increases the magnetic quality of the end product.

It is further known that pressing in a magnetic field can be done either wet or dry. At present, the wet-pressing method is principally used, because in a liquid suspension the particles are more easily aligned than when they are in the dry condition and therefore, the performance values of magnets made by wet pressing are higher than those of dry-pressed magnets. On the other hand, wet pressing is more expensive than dry pressing because the removal of the suspension liquid during pressing is time consuming.

The present invention is based on the discovery that a barium or hard ferrite powder with individual particle diameters between 0.2 and 1 micron, and which have a tendency to agglomerate, can be loosened and made flowable by subjecting the powder to an A.C. field. The A.C. field breaks up any agglomerates which have formed and prevents the formation of new agglomerates. The small grains are spontaneously magnetized providing single magnetic domains with a high coercive force. They, therefore, when subjected to an A.C. field of normal line frequencies (50 or 60 cycles), rotate in synchronization with the field. It is necessary only that the torque moment coming from the A.C. field is stronger than the friction between the particles. This friction effect can be overcome by providing an A.C. field strength of about one thousand oersteds and by pretreating an absolutely dry powder to loosen the same in a micronizer or a cage mill.

In the step of treating in the A.C. field, air which is entrapped by the powder particles becomes intensely agitated during the rotation of the particles; therefore a large increase in volume takes place.

The term hard magnetic of ferromagnetic ferrite will be understood to mean a ferrite with a high coercivity and a high remanence. Examples of such ferrites are those of the formula Mo.6'Fe O wherein MO is a bivalent metal oxide of the group barium, strontium and lead oxides, and mixtures of such oxides.

The invention will be more clearly understood by reference to the following specific example thereof when considered in connection with the accompanying drawing in which:

2,984,866 Patented May 23, 1961 The figure discloses an apparatus for carrying out the process of the invention.

The apparatus shown comprises a hopper 1 for holding a substantial supply of the powder 10 to be molded, and a powder conduit 2 which in this apparatus takes the form of a flexible pipe or tube. The lower end of flexible pipe 2 connects to a filling shoe 3. The hopper 1 has a coil 4 spirally wound about the same, the tube 2 has the coil 14 wound thereabout and the filling shoe 3 has a coil 24 wound thereabout. Suitable connections (not shown) supply an alternating current to coils 4, 14 and 24. The said coils 4, 14 and 24 may all be connected together as one coil or may be separate coils, each supplied with 50-60 cycle alternating current, for example. Alternating currents of lower or higher frequencies may be employed but the 50 or 60 cycle currents are readily available and are adequate.

To avoid eddy currents and other complications due to magnetic fields or other electrical phenomena the parts 1, 2 and 3 are constructed of materials of high electrical resistance. For example, all these parts may be made of plastics or parts 1 and 3 may also be made of laminated low conducting metals such as brass.

The mold 6, as shown, comprises a die cavity 15 surrounded by the cylinder 16, the bottom of which is closed by plunger 17 which is movable axially in said cavity 15. A suitable male mold member, not shown, is provided to compress the powder introduced into the cavity 15 against plunger 17 after the mold cavity is filled with powder and after the mold is moved away from the filling station. The mold as shown can be fitted into the opening 18 of a table 20 along which the filling shoe may be slid. The table 20 is adapted to shield the coil 5 which surrounds the cylinder 16. Coil 5 is connectable by a suitable means (not shown) including a switch, for example, to a source of direct current by suitable terminals not shown. Coil 5 is constructed to provide, inside the filling shoe, when energized, at least twice the magnetic field strength of the coil 24 of shoe 3. For example, the coil 5 may be constructed to produce a field strength of about 2000 oersteds or more. The mold 6 may be constructed of electrically conducting but nonmagnetizable metals, such as bronze, brass or stainless steel.

In operation, the coils 4, 14 and 24 are continuously supplied with alternating current so that powder 10 is free of agglomerates, is free flowing and is somewhat expanded due to the air currents 'built up by the oscillating grains of powder. The filling shoe 3 is moved over a die cavity 15 while the coil 5 of mold 6 is unenergized. When shoe 3 is in position over mold 6 the coil 5 is switched on. When the coil 5 is energized the particles in shoe 3 cease their oscillating motion and assume a motionless oriented position with respect to each other and with respect to the field of coil 5, and the particles are drawn into mold cavity 15 by the stronger D.C. field. The preferential direction of the particles in the mold cavity 15 is in the direction of arrow 7, i.e., the direction of the magnetic field. During the compression step which follows this alignment of the particles becomes fixed.

Example Barium ferrite particles 10 of a size 0.2-1 micron are fed to hopper 1 of the drawing. Alternating current is supplied to coils 4, 14 and 24 and the powder is filled into a mold 6 as described above. The powder is compressed at a pressure of about 9000 pounds p.s.i. (pounds per square inch) and the pressed product is removed from mold 6 and fired at 1200 to 1280 C. Since no suspension liquid has to be removed during pressing the process is much faster than the wet molding process.

3 ..The properties obtained by the process of the invention as compared with the properties of ferrites obtained by other processes;areasifollows:

:Ferritez. 7

(1) Barium ferrite prepared as above 3.3 to 36x10 gauss oe'rsteds.

7 Energy value Other types of hard ferrites act substantially as the barium ferrites described in the specific example.

The features and principles underlying the invention gauss oers teds.

- described above in connection with specific exemplifica- .tions will suggest to those skilled in the art many other modifications thereof. It is accordingly desired'that the appended claims shall not be limited to any specific feature or details thereof.

' I claim:

1 1. In the production of molded and fired, hard magnetic bodies the steps' comprising providing a finely di- -vided dry, loose powder'of the hard magnetic ferrite to be molded, feeding said powder towards a mold cavity, subjecting said powder before it enters said mold cavity to an alternating current field adapted. to rotate said magnetic particles in synchronization therewith, transferring thepowder to the mold cavity and simultaneously aligning the particles as they move into said mold cavity by applying a direct current field about said mold cavity of at least about twice the strength of said A.C. field, thereafter compressing said powder while the powder particles 5 are still aligned and firing the resultant product.

2. As -a molding device for preparing molded pieces from hard-ferromagnetic-ferrite powders the combination comprising hopper means for receiving the molding powders, a filling shoe fortransferringthe powders to a mold, conduit means connecting the hopper with the filling shoe, said filling shoe having an opening opposite to the connection with the conduit means for'communicating with a mold cavity, at least one electrical coil surrounding the conduit means and the hopper, means connecting the coil to a source of alternating current and a mold comprising an axial cavity with an opening at one end thereof, a coil surrounding the mold with the axis thereof corresponding to the axis of the cavity, and means for connecting said last named coil to a source of direct current.

3. A molding device'as claimed in claim 2 wherein an electrical coil supplying alternating current surrounds the filling shoe.

v References Cited in the file of this patent UNITED STATES PATENTS 1,930,788 Buckner Oct. 17, 1933 2,064,773 Vogt Dec. 15, 1936 2,437,127 Richardson Mar. 2, 1948 2,742,185 Landry Apr. 17, 1956 2,873,048 Gear Feb. 10, 1959 FOREIGN PATENTS 558,366 Canada June 3, 1958 1,054,188 Germanyv Apr. 2, 1959 

